Such tool holders generally have a main tube like body with an open forward end within which a chisel tool or drill bit may be inserted. The tool or bit is generally formed with at least one axially extending closed groove towards the rearward end of the tool shank. The tube like body of the tool holder is generally formed with at least one through hole, within the or each of which is located a locking body. The locking body is arranged so that it can be radially displaced between a radially inward locked position and a radially outward unlocked position. The locking body is generally held in the locked position by a locking ring of the tool holder, and in the locked position the locking body engages within the closed groove of the tool or bit. The closed groove is generally axially longer than the locking body and so the tool or bit is locked in the tool holder main body so as to be able to reciprocate by a limited amount with respect to the tool holder body. In the unlocked position, the locking body can disengage the closed groove in the tool or bit and a tool or bit can be removed from or inserted into the tool holder.
In addition to the at least one axial groove closed at both ends, the tools or bits insertable in such a tool holder generally have at least one further axial driving groove offset with respect to the axial closed groove or grooves. The further driving groove or grooves are each open at the rear end of the shank of the tool or bit and when the tool or bit is inserted into the tool holder the driving groove or grooves are each engaged by an axial driving rib of the holder body, by means of which the torque for rotation of the tool or bit is transferred to the tool or bit. Such drill bits are known, for example, as so-called SDS-Plus and SDS-Max drill bits.
The axial driving ribs of the tool holder are generally formed on the tool holder and extend radially inwardly of the inner surface of the tool holder main body. When the hammer to which the tool holder is fitted is used in a hammering mode, the tool or bit reciprocates and so the axial driving groove, which engages the axial driving rib reciprocates with respect to the driving spline causing a considerable amount of wear, especially in a rotary hammer mode in which torque is transmitted from the driving rib to the driving groove. The wear means that the tool holder may have to be replaced by a new tool holder after prolonged use of the tool holder.
This problem has been addressed in U.S. Pat. No. 5,700,018 by replacing the driving ribs with driving elements shaped as rolling bodies, such as cylinders, which rolling bodies are mounted within the tool holder main body so that a portion of the rolling body extends radially inwardly of the inner surface of the tool holder main body. The rolling bodies are arranged to engage corresponding driving grooves of a tool or bit in order to transmit torque. The rolling bodies are mounted within the tool holder body in such a way that they are able to roll along the driving grooves of the tool or bit as the tool or bit reciprocates within the tool holder. This rolling movement of the rolling bodies reduces wear on the rolling bodies and so extends the life of the tool holder. A problem with this is that a non-standard tool or bit is required with driving grooves specially shaped to engage the rolling bodies.
The same problem has also been addressed in EP335,795 and DE199 58 342. In EP335,795 a driving rib insert of hard material is fitted into a corresponding recess in the tool holder body. The recess extends radially from the inner to the outer surface of the tool holder main body and is formed as rectangular opening with outwardly tapering sides. The tapering of the recess prevents the insert from falling into the inside of the tool holder body. The insert is locked in place by a locking ring of the tool holder. The insert is formed with an axially extending driving rib, which when the insert is fitted into the recess, extends radially inwardly of the inner surface of the tool holder body. Thus, the rib is engageable with the driving groves of a standard type of tool or bit. While EP335,795 provides hard wearing driving ribs, the recess in the tool holder main body is of complex shape and relatively difficult to machine.
In DE199 58 342 a driving rib insert of hard material is fitted into the tool holder main body by machining in the walls of the tool holder main body recesses extending axially along the body, which recesses communicate with the central opening of the main body. The insert is then slideably inserted into the axially extending recesses. The insert is formed at its rearward end with a driving rib which extends radially inwardly of the inner surface of the main body so as to be engageable with a driving groove of a conventional tool or bit. The insert is fixed in place by for example, welding, adhesion or soldering. Again, DE199 58 342 provides hardened driving ribs, however, the axially extending recesses undermine the structural strength of the forward end of the tool holder main body. Also, the driving insert is subject to axial vibration due to the relative reciprocation between the driving groove and the rib, which can weaken the fixing between the insert and the tool holder body. The driving insert is a relatively large part, and as it is made of carbide material then it is relatively expensive.